Brannon et al. U.S. Pat. No. 4,508,749 issued Apr. 2, 1985 illustrates that ablation of openings in polyimide can be done using imaging systems in combination with excimer lasers. FIGS. 1 and 2 of this patent illustrate apparatus for accomplishing this purpose. The mask used for ablation in Brannon et al. is typically a reflective dielectric mask. A dielectric mask is required since the usual hard chrome masks quickly erode when exposed to the fluence levels required for polyimide ablation.
More resilient and better suited for the purposes of ablation are the wholly transparent masks of our patent applications Hunter et al. U.S. patent application Ser. No. 08/058,906 filed May 6, 1993, entitled Apparatus and Process for the Production of Fine Line Metal Traces (now U.S. Pat. No. 5,364,493, issued Nov. 15, 1994); Smith et al. U.S. patent application Ser. No. 08/230,183 filed Apr. 20, 1994 entitled High Power Phase Masks for Imaging Systems (now U.S. Pat. No. 5,482,801 issued Apr. 20, 1994); and Smith et al. U.S. patent application Ser. No. 07/833,939 filed Feb. 10, 1992 entitled High Power Phase Mask for Imaging Systems (now U.S. Pat. 5,328,785 issued Jul. 12, 1994).
In certain applications, notably ink jet nozzle production, it is desirable to control the exact fluence profile on the workpiece in order to achieve specific wall slopes and shapes. This much is set forth in Smith et al. U.S. patent application Ser. No. 08/215,851 filed Mar. 21, 1994 entitled Apparatus and Process for Optically Ablated Openings Having Designed Profile. This reference requires that the profile of ablation over the area of an opening in a workpiece be carefully controlled.
Existing techniques for varying the intensity level over a mask are rather crude and limited in their scope of applicability. Most commonly, a multiple step process is used to manufacture a dielectric mask with a discrete number of transmission or gray levels. See Burns et al. U.S. Pat. No. 4,684,436 issued Aug. 4, 1987 and entitled Method of Simultaneously Etching Personality and Select. Alternately, it is known to shape holes utilizing a wobbling optical reflecting surface which imparts to an ablating beam a profile which can be reproduced. Others have noted that certain beams produce holes of certain shapes--and where the shape is desirable, the beam structure is used. It is to be noted that such techniques do not lend themselves to the design of the specific profile of an aperture to be produced. That is to say, determining first the specific profile that it is desired to achieve and thereafter designing a beam and mask combination which will ablate that desired profile--both in overall shape and desired depth.